Multilayer systems applied on a substrate or a carrier, usually consisting of a sequence of different functional and/or intermediate layers, can be used for a multitude of technical applications . For example, in such a construction, optical devices with variable transmission or reflection can be produced in which the reflectance or transmittance can be varied as a function of an applied voltage, making use of the phenomenon of an electrochemically induced spectroscopic change in a material, usually a change of color, also known as electrochromism.
The phenomenon of an electrochemically induced spectroscopic change in a material, usually a change of color, is usually called electrochromism. Making use of this phenomenon, for example optical devices with variable transmission or reflection can be produced in which the reflectance or transmittance can be varied as a function of an applied voltage.
In such electrochromic elements, an electrochromic material interacts with guest ions or electrodes introduced into the electrochromic material by applying an electric potential, the interaction of the electrochromic material being influenced by means of the incidence of electromagnetic radiation. Typical examples of such electrochromic materials are WO3 and MoO3, which, when applied in thin layers on a substrate, are almost colorless. If protons migrate into such a layer, for example in the case of tungsten oxide (WO3), a reduction to blue tungsten bronze will take place, the intensity of the coloring being determined by the amount of the electrical charge flowing in the layer.
Electrochromic elements produced on the basis of this principle of electrochromism can be used in various devices in which a specific variability of the reflection or transmission characteristics is desirable. Such devices can be used, for example, in windows and canopy tops, in particular for motor vehicles. Such applications are, however, particularly advantageous for producing rearview mirrors with a low glaring effect for motor vehicles, because one problem of such rearview mirrors, which are usually designed for a particularly high reflectance in view of a good perceptibility in daylight, consists in the fact that at night, they can be very disturbing, in particular in view of a possible glare through following vehicles. Therefore, in view of the spectral distribution of the light emitted by the headlights of motor vehicles, an efficient antiglare behavior can be achieved in a particularly advantageous manner through an appropriate variation of the reflection characteristics of a rearview mirror by coating such a mirror with a suitably selected electrochromic material. In this case, on the one hand, a particularly high reflectance in daylight can be maintained, while in case of need, i.e. for example after a sensorically detected light incidence at night, the spectral reflection characteristics can specifically be modified by applying a control voltage, thus achieving an antiglare effect.
Especially in view of a possible use in vehicle rearview mirrors, it is desirable that such electrochromic elements achieve with relatively short switching times a particularly high so-called reflection stroke, namely the difference between the reflection maximum and the reflection minimum. In this way, the achievable reduction of the glaring effect by applying the control voltage will be particularly high. Furthermore, electrochromic elements suitable for use in vehicle rearview mirrors should in general have a particularly long service life, in view of their global duration of use and the number of switching cycles.
These requirements are fulfilled to a particularly high degree by an electrochromic element of so-called solid-state construction, known for example from DE 196 40 515 A1. In this electrochromic element, a multilayer system is applied on a transparent substrate, for example on a glass substrate, the multilayer system comprising an ion storage layer applied on a transparent solid-state electrolytic layer, which is applied to the substrate. On the latter, an electrochromic electrode layer is applied, which is covered by a reflective layer. With this construction, in particular a high reliability and operating-time stability can be achieved, which are necessary especially for applications in the area of motor vehicles. In order to guarantee at the same time a marked antiglare effect, a high reflection stroke is desirable especially for this type of electrochromic elements.
In such systems, like in a large number of other technical applications, it can be necessary or at least advantageous to produce at least one of the layers of the multilayer system with a certain porosity. The porosity is usually given according to the so-called Bruggemann model by the volume fraction of pores or spaces not filled with the material of the respective layer by an allocated percentage, a porosity of, for example, 30% according to this definition meaning a volume fraction ratio in the material of the respective layer/air of 70/30. Especially for applications in an electrochromic element of the above-mentioned type, for example the adjustment of a sufficiently high porosity in the solid-state electrolytic layer can be particularly favorable or desirable, as this may considerably favor the ionic conductivity of this layer and, therefore, the rearrangement processes necessary for the desired variability of the reflection or transmission characteristics.
It turned out, however, that in particular in multilayer systems of the above-mentioned type, in which at least one of the layers shall have a relatively high porosity, the adherence of adjacent layers on each other can be problematic. Especially when combining relatively porous layers with relatively compact layers, delaminations may occur in the multilayer system, impairing or jeopardizing the operativeness of the multilayer system as a whole. Such delaminations may also occur in the form of local damages in the respective interfaces between the individual layers, which may result, in particular already in the visual aspect, in grit-like appearances of the surface. It turned out that the adherence between adjacent layers can be improved by keeping the differences in porosity particularly low, which results, however, in a limitation of the possibilities of providing layers with the desired porosity. This can lead to undesired limitations in the working or material characteristics of the multilayer systems produced in this way.